Conventionally, for joining of steel plates to be used in the car body of motor vehicles and so forth, the steel plates are spotwelded to each other, and the joint zone is applied with a sealing material of viscous resin so as to close the gap between the steel plates in the joint zone. In the sealing operation, a gun is moved by hand along the joint zone with the sealing material discharged through the nozzle of the gun. Otherwise, the sealing operation utilizes a robot which moves following a set program for the sealing operation.
In such manual sealing operation, the nozzle of a gun is brought into contact with a sealing-application work (the joint zone between steel plates) so that the gun can be moved following to the feature of the sealing-application work with the aid of visual observation of the work of the steel plates and also with the aid of feeling of a touch to the sealing-application work through the gun. Accordingly, typically as shown in FIG. 7, a gun nozzle 8 can be moved in contact with the joint zone of two steel plates 1, 2 joined through spot welded portions 3, 4, following the features of the joint zone having convex portions 5, 6. Thus manual sealing-application is carried out by a contact way. In this case, the gun nozzle 8 employs a relatively large aperture, and a sealing material 10 is discharged through the gun nozzle 8 directly to the sealing-application work. Accordingly, the shearing speed of the discharged sealing material is relatively low, which enables the joint zone applied with the sealing material to have an aesthetical appearance.
On the other hand, in the above-mentioned sealing operation by use of a robot, it is impossible for the robot to meet all situations of the joint zone as flexibly as the manually operated gun. Even a slight error in the accuracy of the convex portions 5, 6 interferes the movement of the gun of the robot. For elimination of such interferences in the robot sealing operation, it has been attempted to position the gun nozzle 8 at a distance to the sealing-application work 1 and 2. Therefore, the discharge rate of the sealing material needs to be increased as compared with that in the manual sealing-application. For this purpose, a nozzle having a relatively small aperture is employed for the gun.
Namely, in the robot sealing operation where the gun is moved by the above-mentioned non-contact way, the sealing material of viscous resin discharged through the nozzle having a relatively small aperture is made to run through the space from the gun nozzle to the sealing-application work. This requires the shearing speed of the sealing material to be considerably increased at the discharge as compared with that in the above-mentioned manual application by a contact way. In use of a sealing material having such a low viscosity at high shearing speed as that of a sealing material used in the manual application by a contact way, the joint zone applied with the sealing material is deficient in appearance and sealing-property.
In the case that a broad joint zone is applied with a sealing material, the gun is positioned at a distance to the sealing-application work 1 and 2, at an inclination (.theta.) of the gun nozzle to the advancing direction, as shown in FIG. 4. Then, the gun is advanced while the sealing material 10 discharged through the gun nozzle is blown against the work to form a broad application zone. In use of a sealing material having a relatively low viscosity at the high shearing speed for application to a broad joint zone, the sealing material is splashed against the sealing-application work, often resulting in formation of an exposed portion of the beads 13a where the sealing material is excessively spread, as shown in FIG. 6, and the joint zone edges are insufficiently covered with the sealing material. For elimination of such problems, it has been attempted simply to decrease the viscosity of the sealing material. In the case of a broad joint zone applied with such a low viscous sealing material, the sealing-application properties are improved. However, in application of such sealing material to a narrow joint zone, the beads of the narrow joint zone lack in the surface-smoothness, undesirably.